What's on my mind is consciousness. The older you get the more you should think about it, not so much think but explore. Sorry for saying SHOULD. Wait, no. I'm not sorry. The only reason I said sorry is because I thought it's not correct to tell people they should or shouldn't think this that or the other but we do it all the time don't we? If it's incorrect then I admit to being frequently incorrect. So?
Consciousness is on my mind because that's the most mind-blowing thing which can possibly be on anyone's mind. Whether consciousness is in the brain or the brain is inside consciousness or some other explanation wraps it all up, no subject is more exciting, as I see it.